WO1990010196A1

WO1990010196A1 - A sensor element intended for a gyro

Info

Publication number: WO1990010196A1
Application number: PCT/SE1990/000112
Authority: WO
Inventors: Jan Söderkvist
Original assignee: Ab Bofors
Priority date: 1989-02-27
Filing date: 1990-02-20
Publication date: 1990-09-07
Also published as: JP2977136B2; SE466817B; SE8900666D0; CA2047727A1; EP0460089A1; DE69008165T2; EP0460089B1; DE69008165D1; US5251483A; JPH04504617A

Abstract

A sensor element (1') for gyro is disposed with its first end (25) freely vibratable in two planes (the X-Y plane and the Y-Z plane) perpendicular in relation to each other. At its end, the sensor element is fixedly retained in a structure rotatable about its longitudinal axis which is located in the above-mentioned two planes. The first end is vibratable and the vibrations are piezoelectrically detectable by means of drive and sensor electrodes respectively (26, 27, 32, respectively). The sensor element consists of or includes a tuning fork which, at the bottom, displays a transitional portion (23) which merges in an anchorage portion (7'). The electrodes cover substantial parts of the tines of the fork. Contact islets are connected to the electrodes by the intermediary of conductors (32).

Description

A SENSOR ELEMENT INTENDED FOR A GYRO

TECHNICAL FIELD

The present invention relates to a sensor element in accordance with the preamble of appended Claim 1.

BACKGROUND ART

It is previously known in this art to utilize rotating masses as sensor elements for gyros. However, such gyros are technically complex and, in recent years, various types of gyros based on torsion and vibration have, therefore, been designed.

Torsion-sensing gyros normally utilize vibrating tines connected a transitional portion which, in turn, is secured to its ambient surroundings. The transitional portion plays an active part in th torsion oscillation generated by Coriolis forces. An example of such a torsion-sensing gyro is described in US-PS 4,524,619.

Vibration-sensing gyros are based either on a vibrating cylinder structure or on vibrating elongate bodies. For these types of gyros, the transitional portion primarily has a vibration-insulti effect. SUMMARY OF THE INVENTION

TECHNICAL PROBLEM

There is a need in this art for gyros with sensor elements which display high g-force resistance. It is of the utmost importance that the various parts of the gyro withstand severe environmental stresses, in particular high acceleration stresses when the gyro i used in, for instance, ammunition units (missiles, projectiles, grenades etc.) with high muzzle velocities. The gyro must allow fo a rubust construction in which the gyro and its sensor elements ma be of lightweight design in relation to the weakest point of each respective structure. It is essential, in respect of ammunition units, that the gyro may be capable of emitting an output signal even under the acceleration loads which apply. There are also ofte demands on small external volumes.

SOLUTION

The major object of the present invention is to propose a sensor element for gyros which affords a solution to the above outlined problems. That which may essentially be considered as characte¬ rizing the novel apparatus according to the present invention is apparent from the characterizing clause of appended Claim 1.

The novel sensor element is preferably designed as a tuning fork o quartz crystal. The base of the tuning fork is fixedly secured, while the tines of the tuning fork are allowed to vibrate freely. Drive electrodes are deposited on the crystal, these causing the tines of the crystal to vibrate in desired directions, with the ai of the piezoelectric effect. From the base of the crystal, it is possible to connect conductors (for example with the aid of wire bonding or TAB) and, by such means, establish contact with the surrounding electronics. The sensor element is of a design which makes for production usin conventional technology in quartz oscillator manufacture. The resonance frequencies of the tines of the sensor element may be adjusted by coating different parts of the tines with a mass, for example gold. Alternatively, the removal of mass may be employed, in which event such removal is ideally carried out with the aid o a laser. Mounting of the crystal in its associated capsule may be effected pursuant to prior art technology, using, for instance, glue.

Preferably, the electronics associated with the above-disclosed sensor element are placed as close to the sensor element as poss¬ ible, since a quartz crystal is extremely sensitive to capacitive disturbances. Consequently, hybrid electronics are to be preferre which makes it possible to keep the volume of the gyro to a minim The size of the sensor element need not be greater than that of a standard conventional watch crystal. The major portion of the volume will be taken up by the electronics, even though this is o hybrid design.

ADVANTAGES

In addition to solving the above-outlined problems, the novel sensor element for gyros according to the present invention may b manufactured in economically favourable manufacturing processes. The gyro as such will be operationally highly reliable and robust in that it will withstand such factors as high acceleration force

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

One currently proposed embodiment of a sensor element displaying the characteristics significative of the present invention will b described in greater detail below with reference to the accompany ing Drawings. In the accompanying Drawings:

Fig. 1 is a horizontal view showing a tuning fork sensor element; Fig. la shows the orientation axes X, Y and Z for the element according to Fig. 1;

Fig. 2 is a side elevation of the tuning fork according to Fig. 1; Fig. 2a shows orientation arrows for the above-mentioned X, Y and Z axes;

Fig. 3 is a vertical view showing the design of one tine of the tuning fork according to Frg. 1;

Fig. 4 shows, in perspective, example of a fork design in which the sitings of the drive and sensor electrodes on the one tine are illustrated;

Fig. 4a is a schematic view showing the piezoelectric phenomena which occur in one tine on activation of the drive electrodes according to Fig. 4;

Fig. 4b is a schematic diagram showing the piezoelectric phenomena for the sensor electrodes on the tines;

Figs. 5-7 illustrate one practical design of the drive and sensor electrodes;

Figs. 8-10 show an alternative embodiment of the electrode applica¬ tions in relation to that illustrated in Figs. 5-7; and Figs. 11-12 show yet a further embodiment of the electrode applica¬ tion.

PREFERRED EMBODIMENT

Referring to the Drawings, Fig. 1 shows the fundamental makeup (the geometry) of a sensor element 1. The element includes two tines 2 , 3. In this case, the cross-sectional area of the tines is constant throughout the entire lengh of the tines. However, it is also possible to design the tines with varying cross-sectional area. The tines are fixedly connected to the base 4 of the tuning fork. This consists, for the greater part, of a rectangular structure. The entire sensor element with the tines 2, 3 and the base 4 is produce from a single piezoelectric piece. By waj_^ of suggestion, use may be made of prior art quartz crystal technology for its production, which implies that the tuning fork will be given constant thicknes for both the tines and the base according to Figs. 2 and 3. Recess 5, 6 or corresponding projections may be introduced on the base fo achieving vibration insulation of the anchorage surfaces from the vibrations of the tines.

Crystallographically, it is to be preferred that the tines point along the piezoelectric mechanical axis, the Y axis in Fig. la. Th thickness of the tuning fork should extend in the optical axis, as constituted by the Z axis in Figs, la and 2a. The width of the tines will then be deposited in the electrical direction, along th X axis in Figs, la and 2a. The X and Z axes can also change place, but in the remainder of the body of this specification, it will be assumed that the tuning fork lies in the X-Y plane. Minor varia¬ tions in water cutting angle and orientation of the tuning fork ma be preferable in order, for instance, to influence temperature properties and mechanical coupling between the eigenmodes of the tuning fork.

The tuning fork is coated with a suitably selected electrode configuration in order to make possible excitation of a vibration of the tines in the X-Y plane. Sensor electrodes for sensing a tin vibration in the Y-Z plane should also be deposited on the tines. Output islets/bond pads/contact material utilized for establishing electric contact with the ambient surroundings is deposited on the base of the crystal. The electrodes and their output islets are connected by means of conductors on the tuning fork. 'The applica¬ tion of the electrodes, conductors and output islets may be effec¬ ted employing prior art technology from quartz manufacture.

The crystal is mounted via an anchorage portion 7 on a securement device 8, for instance by gluing. Naturally, the anchorage portion may be fixedly retained by other means than gluing. That part of the base of the tuning fork which is counter-directed in relation to the tines is ideally employed as gluing surface. Alternatively, the sides of the base could be employed as gluing surface. The output pads are electrically connected to the surroundings with the aid of wire bonding, TAB (Tape Aided Bonding) , conductive glue or the like. It could be an advantage in the electrical connection if all output terminals are located on one side of the base. One alternative method of mounting the crystal in place is to clamp the anchorage portion of the base fixedly between two plates.

The design of the base is preferably selected in such a manner that the vibrations of the tines do not influence that portion of the base which is employed for mounting. For example, the output islets should be placed on or in the proximity of that part of the base which is employed for the anchorage, in order that bonding wires etc. do not have a damping effect on the oscillatory movements of the tines. If the base were to participate in the vibration of the tines to an unacceptably high degree ('parasitic oscillation'), there is a risk of energy losses within the mounting area. More¬ over, external vibrations could easily reach the tines and thereby generate a false gyro signal.

In order to minimize the effect of ambient vibrations on the vibration of the tuning fork - and in particular on the output signal - it is of the utmost importance that the tuning fork and its anchorage be designed as symmetrically as possible.

The output signal from the sensor element or sensor derives from the following physical phenomena; using suitable electronics and electrode patterns, a vibration is generated in the tines, in the X-Y plane. The amplitude of this vibration is kept constant. Its frequency is selected equal to one of the resonance frequencies of the tines. The frequency selected should imply that the tines oscillate in opposition, i.e. the end points of the tines move towards one another during half of the period time and away from one another during the other half. When the tuning fork is rotated about the Y axis, Coriolis forces will occur which generate a driving force which strives to excite the vibration in the Y-Z plane. That vibration which is generated in the Y-Z plane will hav an amplitude which is directly proportional to the impressed rotation velocity. Just as in the vibration in the X-Y plane, the tines will, in this vibration, move in opposition to one another. The resulting vibration in the Y-Z plane is detected with the aid of electrodes which are sensitive only to vibrations in this plane By signal processing and demodulation, a DC signal will be obtaine whose size is directly proportional to the impressed rotation. Alternatively, the resulting vibration may be detected electrostat cally.

In order to obtain a large output signal, it is of the utmost importance to select resonance frequencies for the two vibrations which are almost identical with one another. The reason why coinci ding resonance frequencies is not desirable is that temperature variations would imply that the resonance frequencies drift somew from one another. These variations are small, but sufficient to change the phase of the sensor vibration in an unacceptable manne This may be remedied by chosing the resonance frequencies to diff to such an extent that temperature variations have no appreciable effect on the sensor vibration.

Manufacturing inaccuracy for the tines will result in an inaccura in the resonance frequencies which is greater than the desired frequency difference between the two utilized resonance frequenci Hence, adjustment of the resonance frequencies will be warranted such that the desired frequency difference is obtained. This adjustment may be effected in that mass is added to or removed fr appropriately selected points on the tines. The technology for carrying out such application or removal of mass, respectively, m be considered as well known to a person skilled in this art. Sinc the frequency difference is to be adjusted, such balancing should preferably only influence one of the two resonance frequencies. F example, mass may be applied at a point where the vibration of on of the resonance frequencies has a node. Since the vibration out the plane affects the base of the tuning fork more than does the vibration in the X-Y plane, a suitable region from this point of view is the transitional area between base and tines. In this balancing, mass should be applied symmetrically on the tines in order that no imbalance occurs.

Those electrodes 9, 10, 11 and 12 which are employed to cause the tines of the crystal to vibrate in the X-Y plane are connected to suitable drive electronics. Such electronics applies a sinusoidal electric voltage across the drive electrodes. The electronics are adapted in relation to the crystal such that the frequency of the signal agrees with a suitable resonance frequency. One proposed electrode configuration for this vibration is depicted in Fig. 4a. This configuration generates a field 13 in the cross-sectional area of the tines. Since a piezoelectric material such as quartz is insensitive to fields in the Z axis, the generated field in the above-outlined electrode configuration may be assumed to be equiva¬ lent to an electric field 15, 16 illustrated in the cross-section 14. This latter field generates an elongation strain in the Y axis in one portion of the tine and a shrinkage in the other. The tine will then strive to bend in the X-Y plane. Since the electrical supply voltage is time-dependent, the field in the cross-section of the crystal will also be time-dependent, and thereby the movement of the tines of the tuning fork. The vibration in the X-Y plane is generated in this manner.

In a manner corresponding to the vibration in the X-Y plane, an electrode configuration 17, 18, 19 and 20 may be employed which is only sensitive to vibrations out of the plane of the tuning fork. These electrodes 17-20 are utilized to sence a time-varying deflec¬ tion in this vibration direction. When vibration exists, the piezoelectric crystal structure will be deformed. This results in surface and volume changes being generated. These charges will, together with the proposed electrode configuration, create a filed pattern 21 in the cross-section 22. This field pattern causes electrons to migrate to and from the electrodes. A current whose size is dependent upon the relevant vibration amplitude in the Y-Z plane will thus be created. The illustrated electrode configuration constitutes but one exampl Other electrode configurations than those described in the fore¬ going may be employed to obtain the desired function.

When the crystal is mounted, there will, because of shortcomings i manufacture, be a coupling between the two vibration directions. This results in an output signal being present from the sensor electrodes even if the sensor is not subjected to any rotation. This "cross-talk" is undesirable, since it impairs- he performanc of the gyro (for example as regards temperature stability) . One method of eliminating this link between the two vibration direc¬ tions is to balance one of the tines in such a manner that the tines will have identical vibration properties. Such balancing is effected, for instance, on the tines in the regions B or B' in Fig. 1.

In order that the sensor be capable of withstanding extremely severe mechanical stresses such as powerful acceleration, its structure must be robust. Thus, the structure may not include any flimsy details. All unnecessary parts of the structure should be removed. Those parts which must be included are the tines, in ord to be able to generate the reference vibrations (in the X-Y plane) which makes possible the Coriolis force. These tines must be anchored somewhere. Hence, the base of the sensor is also indispe ible. This vibration which the Coriolis force generates must also be sensed. This may be effected by integrating a mechanical part with the sensor (torsion sensing) . Since the number of parts of t sensor is to be kept to a minimum, it is, however, to be preferre if sensing of the vibration can essentially be effected out on those tines which are employed to generate reference vibrations, those regions where the mechanical stress, caused by the sensor vibration, is greatest.

To be able to utilize the tines for both driving and detecting, i is necessary that both of the electrode configuration required fo these two vibrations share the space available on the tines. The electrodes must then be optimated to the available space. Electro¬ des far out on the tines have a low level of efficiency, while electrodes further in on the tines have a higher level of effici¬ ency. If the drive electrodes and the sensor electrodes would be allowed to cover identical areas, then the amplitude in the drive direction would be largest. If good performance is required of the electrodes utilized for the electrical driving, in order to simplif the requirements placed on the drive electronics, these electrodes must be placed proximal to the transition between base and tine. If it is required that performance of the two electrode configurations should not show an excessive mutual discrepancy, the sensor electrodes should be placed near the transitional portion 23, 23' between base and tine.

The size of the drive electrodes is then selected such that desired performance is obtained in respect of the electronics. This elect¬ rode siting is to be preferred, not least because the sensor electodes may then also be permitted to cover a portion of the transitional portion between base and tine and thereby make use of the fact that a portion 23, 23' of the base is actually vibration- ally active during the sensor vibration of the tines.

An electrode pattern which relates to the above-described division of the tines is included in Figs. 5, 6 and 7. This pattern should only be seen as one conceivable method of realizing the above concept. Figs. 8, 9 and 10 show a corresponding pattern if the drive and sensor electrodes change place. In the first figures 5, 6 and 7, the conductors have been sited with a view to minimizing the number of output islets 24 and placing all output islets on one side of the tuning fork. When the sensor electrodes are placed most proximal the base, the borderline between drive and sensor electro¬ des should be placed at a distance of between 20 per cent and 40 per cent of the tine length from the base. In Figs. 8, 9 and 10, the output islets (connection material) are greater in number. In Figs. 5, 6 and 7, the region of between 30 and 80 per cent of the tine length - counting from the base - is covered with drive electrodes on all side surfaces of the tines. In Fig. 5, two of th drive electrodes have been designated with reference numerals 25 and 26. The drive electrodes are connected to contact islets 24 b the intermediary of conductors 27. In order to minimize disturbanc to and from the conductors, these have been paired together such that the disturbance field emanating from them is of a dipolar nature. The sensor electrodes 28, 29 and 30 are connected to their connection islets by the intermediary of conductors 31. The sensor electrodes are disposed on both sides of the sensor. The contact islets may be disposed either on one side of the sensor or on bot sides thereof.

In Figs. 8-10, the sensor electrodes 32, 33, 34 and 35 are locate furthest out on the free ends of the tines. Two mutually opposing sides of the tines are coated with sensor electrodes, of which on the one side is shown in Fig. 8. In this case, the drive electrod are located at the bottom of the tines and extend down over parts of the transitional portion 23'. In this case, one pair of drive electrodes 36, 37 is employed on each tine surface. The connectio to the contact islets 24 are effected by the intermediary of conductors 38. The contact islets 24 according to Fig. 8 cover large parts of the anchorage portions on the sensor element.

Figs. 11 and 12 illustrate yet a further variation on the locatio of the sensor and drive electrodes on the tuning fork, partly fro the front face (Fig. 11) , and partly from the rear face (Fig. 12)

In this case, the drive electrodes 39, 40, 41 are placed furthest out on the free ends of the tines on both the front and rear face The electrodes 40 and 41 are deposited as side electrodes. The drive electrodes are connected to contact islets 42, 43 and 44, t contact islet 44 being designed as a capacitor. The sensor electrodes 45, 46, 47 are disposed at the bottom on the tines and extend down over parts of the transitional portion 23". The sensor electrodes are placed on both the front and rear face and are connected each with their respective contact islets S+, S- by the intermediary of conductors.

Two dummy conductors 48, 49 for compensation of stray capacitances are disposed on the front face of the tuning fork and extend from the tines in the region between the drive and sensor electrodes to contact islets 50, 51 on the base of the tuning fork.

The above-considered structures are based on the so-called tuning- fork principle. However, there is nothing to prevent a tuning fork from being designed so as to include but a single tine. Its func¬ tion will be identical. Also in such a case, the tines will include electrodes for both driving and detecting. However, the advantage inherent in a tuning fork comprising two tines is that the vibra¬ tions in the two vibration directions are more localized to the tines as compared with if only one tine had been employed. Hence, influence on the gyro from its surroundings will be less if two tines are employed. Naturally, systems including more than one tuning fork may also be designed.

Those resonance frequencies which are utilized are directly propor¬ tional to the resulting mechanical strength of the tines. Thus, the frequency employed may not be too low. If a high resonance frequenc is employed, the sensitivity to external vibrations of the sensor will, at the same time, be reduced.

The present invention should not be considered as restricted to the embodiment described in the foregoing and shown on the Drawings, many modifications being conceivable without departing from the spirit and scope of the appended Claims and inventive concept as herein disclosed.

Claims

1. A sensor element (1) intended for a gyro and disposed with it first end freely vibratable in two planes perpendicular in relatio to each other, designated to X-Y plane and the Y-Z plane, and with its second end fixedly secured in a structure rotatable about its longitudinal axis which coincides with said planes, the first end (25) being vibratable and the vibrations being piezoelectrically detectable by means of drive and sensor electrodes (9-12) , respec¬ tively, c h a r a c t e r i z e d in that the sensor element consists of or includes an elongate body which, in its second end, displays a transitional portion (23, 23') and an anchorage portion (7) , which do not participate in the vibration to any appreciable degree; that said drive and sensor electrodes cover substantial portions of the vibratable end of the elongate body; and that the sensor element is, by means of conductors, coupled to connection material (24) located on the anchorage portion, by means of which material connection to outer electronics is effectuable.

2. The sensor element as claimed in Claim 1, c h a r a c t e ¬ r i z e d in that it includes two elongate bodies included in a fork construction, said bodies being together with the transition portion (23) and the anchorage portion (7F) manufactured in one common piece of piezoelectric material, for example quartz.

3. The sensor element as claimed in Claim 2, c h a r a c t e ¬ r i z e in that the resonance frequencies of the elongate bodies, hereinafter designated tines, are adjusted with the aid o balance mass or masses, for example of gold.

4. The sensor element as claimed in any one of the preceding Claims, c h a r a c t e r i z e d in that it displays a small outer volume, for example with a total length of at most approx. 6 mm and a maximum thickness of at most approx. 0.6 mm. _. .

- 14-

5. The sensor element as claimed in any one of the preceding Claims, c h a r a c t e r i z e d in that the transitional portion is designed with recesses and/or projections (5, 6) which prevent vibration transmissions between the tines and the anchorage portion.

6. The sensor element as claimed in any one of the preceding Claims, c h a r a c t e r i z e d in that each respective elon¬ gate body/tine carries a drive electrode configuration which makes possible excitation of vibrations of the tines in the X-Y plane: and that each respective tine displays a sensor electrode configu¬ ration which makes possible detection of vibrations in the Y-Z plane.

7. The sensor element as claimed in any one of the preceding Claims, c h a r a c t e r i z e d in that it is disposed such that the vibrations of each respective elongate body/tine does not influence or is not transmitted to the anchorage portion (7) ; and that the contact material/connection islets (24) are located on the anchorage portion such that outer connections are prevented from exercising a damping influence on the vibrations in each respective body/tine, so that the risk of energy loss arising out of the outer connection (t) is prevented and/or such that outer vibrations are prevented from being impressed on each respective elongate body/ /tine and giving rise to a false gyro signal.

8. The sensor element as claimed in Claim 7, c h a r a c t e ¬ r i z e in that the design and anchorage of the tuning fork are symmetrical so as to prevent ambient vibration from influencing the vibrations of each respective elongate body/tine.

9. The sensor element as claimed in Claim 3, c h a r a c t e ¬ r i z e d in that the balancing is effected outermost on each respective tine. ..- -

10. The sensor element as claimed in Claim 3, c h a r a c t e ¬ r i z e d in that balancing masses are deposited on the tines at point where one of the vibrations has a node.

11. The sensor element as claimed in any one of the preceding Claims, c h a r a c t e r i z e d in that it is of robust constr tion without flimsy parts.

12. The sensor element as claimed in any one of the preceding Claims, c h a r a c t e r i z e d in that in the event the senso electrodes are placed most proximal the base, the borderline between drive and sensor electrodes is sited at between 20 and 40 per cent of each respective tine length from the base.

13. The sensor element as claimed in any one of the preceding Claims, c h a r a c t e r i z e d in that the resonance frequenc is in excess of 30 kHz.